Prostaglandins regulate vascular tone and salt and water homeostasis in the mammalian kidney and are involved in the mediation and/or modulationof hormonalaction. Cyclooxygenase (prostaglandin synthase G2/H2) is the rate-limiting enzyme in metabolism of arachidonic acid to prostaglandin G2 and subsequently to prostaglandin H2, which serves as precursor for subsequent metabolism by prostaglandin and thromboxane synthetases.lt is now recognized that cyclooxygenase-2 (COX-2) is highly expressed in the macula densa and surroundingcortical thick ascending limb (cTAL) of mammalian kidney and is involved in regulation of renin expression and secretion. Our studiesin primary cultured cTALH have indicated that COX-2 expression is increased in response to decreased extracellular chloride by a p38 dependent process. There are three specific aims in the current proposal.In the firstaim, we will examine mechanisms of regulation of COX-2 expression in macula densa/cTAL. We hypothesize that decreased extracellular chloride increases COX-2 expression in cTAL/macula densa by cell shrinkage and/or by decreases in intracellular chloride, which will activate PI_A2activityand release prostaglandinsthat activate p38 activity. We further hypothesize that COX-2 increases both byNF-kappa B-mediated transcriptional activation and by increased mRNA stability. The second specific aim willinvestigate the modulationof cTAL/macula densa COX-2 expression by the renin-angiotensin system and nitric oxide. We hypothesize that these agonists modulate COX-2 expression by altering chloride flux andthereby modulatingp38 activity. The third specific aim will investigate the modulationof renin expressionby COX-2. In these studies, we will examine the regulation of prostaglandin synthases in high renin states and examine the interaction of COX-2 metabolites and TGFI_2 in regulation of renin expression. We also propose to develop mice with selective deletion of COX-2 in the endothelium to determine the role of endothelial-generated COX-2 metabolites in regulation of renin expression and secretion. In summary, these studieswill providefurther insight into regulatory mechanisms of renal corticalCOX-2 expression and physiologicroles of COX-2-generated metabolites in regulation of the renin-angiotensin system.